1. Field of the Invention
The present invention relates generally to feedback control systems and, more particularly, to phase-locked loops.
2. Description of the Related Art
A phase-locked loop is a particular type of feedback control system that maintains an output signal in a specific phase relationship with a reference signal. Phase-locked loops are vital parts of a wide variety of electronic systems (e.g., frequency synthesizers, analog and digital modulators, clock recovery circuits and direct digital synthesizers) and the basic structure of conventional phase-locked loops has been described (e.g., see U.S. Pat. Nos. 6,222,421 and 6,252,466 respectively issued Apr. 24, 2001 and Jun. 26, 2001).
Conflicting demands are placed on the selection of a loop bandwidth for a phase-locked loop. The loop bandwidth is preferably set low to filter out input-related spurious tones and phase noise to thereby meet required system spectral and noise performances. The loop bandwidth, however, is preferably set high to achieve fast output-signal switching time in response to a frequency change of the reference signal.
The selection of loop bandwidth has therefore typically been a compromise which degrades one or more phase-locked loop performance parameters. Accordingly, there is a need for feedback systems that enhance output-signal switching times without degrading other loop performance parameters.
The present invention is directed to feedback systems that enhance output-signal switching times without degrading other loop performance parameters (e.g., loop spectral and noise reduction).
These goals are realized with feedback control systems that include charge pump and loop filter combinations which reduce xe2x80x9ckick-backxe2x80x9d voltages that are generated in the loop filter by drive currents which rapidly drive a control loop oscillator to a loop acquisition range. It has been found that the kick-back voltage generates a frequency step in the oscillator output signal which must then be driven to eliminate the frequency step with a consequent increase in the output-signal switching time. The systems of the invention reduce the kick-back voltage to thereby enhance output-signal switching times.